1. Field of the Invention
The present invention relates to a toner for electrophotography, and a process for preparing the same.
2. Description of the Related Art
Conventionally, a kneading and pulverizing process has been known as a process for preparing a toner for electrophotography. Commonly, according to this process, components for the toner are kneaded and then pulverized to obtain the toner having a desired particle size. Presently, the toner prepared by this kneading and pulverizing process is widely used. However, as users are requiring higher performances, a superior toner has become necessary.
Presently, the performances sought greatly in regards to electrophotography are cleaner-less performance (which does not generate waste toner) which is closely linked with conservation of resources; and higher speeds, higher image quality and more compact structures of a device, which have been in greater demand with development of personal computers. Demand for these performances are especially intense in regards to color image-outputting devices, because of present circumstances that almost all computers display color images of excellent vividness.
However, the shape of the toner particles obtained by the kneading and pulverizing process is indefinite; therefore, there remains a problem that fluidity of the toner is low and an area of the toner particles contacting a surface of an electrostatic latent image support member is large, thereby decreasing transferability of images. Further, in the kneading and pulverizing process, colorants added to the toner appear on a surface of the obtained toner particles. Thus, an electric charge on the surface of the toner becomes non-uniform, resulting in a problem that a charge distribution of the toner is widened so that developability is lowered. At present, because of these problems, the kneading and pulverizing process cannot sufficiently satisfy the demand for the higher performances.
In order to satisfy the demand for the higher performances, various processes for preparing a toner have been proposed. For example, a toner having a capsule structure and processes for preparing this toner are proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 60-222868, 61-114247, and 59-162562. According to these processes, the toner particles can be made spherical, so that it is possible to obtain the toner having excellent fluidity and transferability. Since colorants or the like do not appear on a surface of the toner particles, the obtained toner also has excellent uniformity in charging. However, the surface of the toner particles is completely covered with a hard shell, and consequently fixing property is low in a hot roll fusing method, which is the main trend at present, making it difficult to obtain a higher speeds and compact structures.
Japanese Patent Application Publication (JP-B) No. 36-10231 proposes a toner having a pseudo-capsule structure and a process for preparing the toner using a suspension polymerization method. In this suspension polymerization method, toner particles can be made spherical so as to obtain excellent fluidity and transferability. Colorants and the like are not liable to appear on a surface of the toner particles so that the obtained toner also has excellent uniformity in charging.
In many cases, however, raw materials used for a toner in the prior art cannot be used because resin is polymerized while the toner is prepared. Even in cases where it is possible to use conventional materials and carry out polymerization, particle size of the toner frequently cannot be controlled sufficiently because of influence of additives such as resin and colorants. Thus, this process has a problem that the raw materials cannot be freely selected. Particularly, there is a problem in that polyester resins, which exhibit excellent fixing property and color adaptability in the kneading and pulverizing process in the prior art, cannot be used in principle. Thus, the toner obtained by this process cannot sufficiently satisfy the demand for compact structures, higher speeds and colorization.
To overcome these problems, methods for preparing a toner by a dissolution suspension process are proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 5-127422, 7-152202, 7-168395 and 7-271099. In the dissolution suspension process, an oil phase wherein toner components are dissolved or dispersed into an organic solvent is suspended in an aqueous phase to prepare toner particles. Since a polymerization process is not involved, a wide range of raw materials can be selected, and polyester resin and colorants can be used, thereby easily making the toner particles spherical and easily controlling particle size. For this reason, the toner having excellent fluidity and transferability can be obtained, and it is possible to realize a cleaner-less device. Since the toner obtained by this process has excellent fixing property and color adaptability and does not generate non-uniformity in charging caused by surfacing of a colorant, it is expected to be able to accommodate the demand for compact structures, higher speeds, and colorization.
However, even in the dissolution suspension process, wherein colorant components hardly appear on the surface of the toner particles, the colorant is liable to agglomerate so that it is difficult to add and disperse the colorant uniformly into the toner. Thus, variation is produced in the incorporation of the colorant among the toner particles, resulting in a problem of generating non-uniformity in charging, and of deteriorating stability in a case in which the toner is used for a long time. Furthermore, in a case of outputting color images, there occurs a problem in which a slight deterioration in developability and transferability impairs color balance and gradation. Further, the colorant in the toner is generally insoluble in the resin of the toner, and consequently transmissive light is irregularly reflected off interface between them, interfering with transmission of light through overhead projection(OHP) transparencies and the like. Thus, when the colorant is insufficiently dispersed, there occurs a problem of impairing the transmittance of light through the OHP transparencies.
Further, in a color image-outputting device in particular, it is common that in a fixing device an oil supplying device is unnecessary and that an oilless toner is used, wherein a releasing agent is added into the toner as a substitute for oil. However, the releasing agent cannot be made into particles as fine as a colorant, and uniform addition and dispersion of the releasing agent are difficult, so that there occurs a problem that chargeability, developability, storage ability and OHP transmittance are impaired when the releasing agent is poorly dispersed in the toner.
Using a flushing pigment as a method for dispersing the colorant is common in the kneading and pulverizing process. In the dissolution suspension process, however, even if the flushing pigment is used, a flushed resin is dissolved so that the flushing pigment cannot produce the aforementioned effect. Thus, dispersibility is not improved. In order to disperse the releasing agent, high shearing force can be easily applied in the kneading and pulverizing process. However, shearing force causing the releasing agent to be sufficiently dispersed cannot be applied in the dissolution suspension process.
Japanese Patent Application Laid-Open (JP-A) No. 5-119529 discloses that in an emulsion dispersing process, which is a process for preparing a toner which similarly involves use of a solvent, a graft resin is used as the binder resin to improve dispersibility of a colorant. To be certain, this process improves dispersibility of the colorant. However, since it is necessary to considerably increase a graft rate of the binder resin, a primary property of the binder resin is liable to deteriorate. This process causes, in particular, fixing property of a color toner and image quality thereof to deteriorate, and is not preferred.
As described above, a toner for electrophotography fully satisfying the demand for higher performances has not yet been obtained.